Monitoring system for a distributed antenna system

ABSTRACT

A monitoring system 22 for a distributed antenna system (DAS) 10 is provided. The DAS comprises central transmitter 12 which is connected by a signal transmission network 14 to a plurality of distributed antenna devices (DAD) 16.1 to 16.n. The network comprises physical branches. Each of the DAD&#39;s is connected to a respective sub-branch 14.11 and comprises at least one antenna 18. The antenna is associated with a frequency band having a center frequency f c  and an associated wavelength Ac. The monitoring system comprises a central monitoring unit (CMU) 24 which is coupled to the network 14. A monitoring device 28.1 is associated with at least one of the DAD&#39;s and permanently mounted a distance d&lt;2λ c  away from the antenna of the DAD. The monitoring device comprises a controller 30, a transceiver 32 and an antenna 34. The controller being configured, upon being polled by the CMU 24 with a monitoring signal via the network and the distributed antenna device, to cause the transceiver 32 to respond by transmitting a response signal to the CMU 24 via the distributed antenna device 16.1 and the network 14.

REFERENCE TO RELATED APPLICATIONS

This application is an application for reissue of U.S. Pat. No.9,900,114, issued Feb. 20, 2018, which issued from U.S. patentapplication Ser. No. 15/504,977, filed Feb. 17, 2017, which is the U.S.National Phase of International Application PCT/IB2015/056343, filedAug. 21, 2015, and claims priority to ZA Application No. 2014/06162,filed Aug. 21, 2014. Each of the priority applications is herebyincorporated by reference in its entirety.

INTRODUCTION AND BACKGROUND

This invention relates to distributed antenna systems, more particularlyto a monitoring system and method for a distributed antenna system.

Distributed antenna systems (DAS) are known in the art and are typicallyemployed to provide in-building coverage, but more recently are alsoused to provide area coverage outside of buildings. DAS may be passiveor active. A passive DAS comprises a central or base transceiver stationlinked by a radio frequency (RF) signal transmission network comprisingRF transmission lines (coaxial or other) to a plurality of distributedantenna devices (DAD) distributed through the area to be covered withpower diverted according to some propagation plan to each DAD, such thatadequate coverage is ensured throughout the building or area for whichthe DAS is installed.

An active DAS is similar, but in these systems the RF signals aremodulated up to convert them to optical fibre frequencies. Opticalfibers are then used to distribute the resulting signals to a pointclose to the DAD. An optic-to-radio converter unit is used to convertthe signals back to their original RF band. The optic-to-radio convertoris coupled using RF transmission lines which provide the last mile toone or more DAD's, which provide coverage to sub-areas of the areacovered by the DAS. Other active DAS systems may involve bi-directionalamplifiers and/or frequency convertors in between the centraltransceiver and the DADs.

A DAS may employ between a few to many hundreds of DAD's to providecoverage throughout the area. These DAD's or the signal transmissionnetwork and intermediate devices used to link these DAD's to the basetransceiver station may fail or degrade over time. Currently such faultsare difficult to detect or monitor. One known solution is to use regular“walk tests” to measure network coverage throughout the coverage area,but these are time consuming, costly and faults are detected well afterthey had occurred.

In U.S. Pat. No. 8,254,848 there is disclosed another solution whichcomprises a plurality of statically deployed monitoring devices. Themonitoring devices are remote from the DAD's being monitored and testresults are reported to a central and remote collection componentdirectly or indirectly through other monitoring devices having anEthernet connection. This solution may be unnecessarily costly.Furthermore, due to the separation between DAD's and the monitoringdevices, individual DAD's and branches in the network may be difficultto pinpoint. Hence, the solution may not be suitable for at least someapplications.

OBJECT OF THE INVENTION

Accordingly it is an object of the present invention to provide amonitoring system and method with which the applicant believes theaforementioned disadvantages may at least be alleviated or which mayprovide a useful alternative for the known systems and methods.

SUMMARY OF THE INVENTION

According to the invention there is provided a monitoring system for adistributed antenna system comprising at least a central transmitter anda plurality of distributed antenna devices connected to the transmittervia a respective physical branch of a signal transmission networkcomprising a plurality of branches, each of the distributed antennadevices comprising at least one antenna which is associated with afrequency band having a centre frequency f_(c) and a correspondingwavelength λ_(c), the monitoring system comprising:

-   -   a central monitoring unit which is coupled to the network;    -   at least one monitoring device associated with at least one of        said distributed antenna devices and permanently mounted a        distance d away from the at least one antenna of the distributed        antenna device, the distance d being less than 2 times λ_(c)        (2λ_(c);)    -   the at least one monitoring device comprising a local        controller, a transceiver and an antenna;    -   the local controller being configured, upon being polled by the        central monitoring unit via the network and the distributed        antenna device, to cause the transceiver to respond to the poll        by transmitting a response signal to the central monitoring unit        via the distributed antenna device and the network.

The distance d may be less than λ_(c), preferably less than λ_(c)/2 andeven less than λ_(c)4.

The monitoring device may comprise a local power supply, for example inthe form of a battery. Alternatively or in addition, the monitoringdevice may comprise an energy harvesting circuit for collecting energyfrom the DAD through the antenna of the monitoring device. The energymay be used to recharge the battery.

Each monitoring device may be associated with a unique address which maybe stored in a memory arrangement of the monitoring device.

Each monitoring device may also comprise indicator means for providing ahuman perceivable indication relating to a monitored status of theassociated DAD and/or the branch of the network connected thereto.

The monitoring device may be mounted in or on the DAD. It may forexample be retrofitted on a radome of the DAD. In such a case, themonitoring device may be housed in a housing or encapsulated in aflexible sleeve or envelope, which may be adhered to the radome.

The central monitoring unit may comprise means for measuring thestrength of the response signal received from the at least onemonitoring device and a database for storing data relating to the uniqueaddresses of each monitoring device, data relating to a monitored statusof each DAD and optionally data relating to the position of the DADassociated with the monitoring device.

The central monitoring unit may be coupled to the network by a suitablecoupler to inject a weak monitoring message or signal or tone into thenetwork. The message or signal or tone may be in-band or out of specificDAS communication bands, but within the overall band which the DASsystem is designed to operate over.

The monitoring message may be addressed to a targeted monitoring deviceby using the unique address of the monitoring device.

The invention also includes within its scope a DAS comprising amonitoring system as herein defined and/or described.

Still further included within the scope of the present invention are acentral monitoring unit as herein defined and/or described and amonitoring device as herein defined and/or described.

The invention also includes within its scope a method of monitoringperformance of a distributed antenna system comprising at least acentral transmitter and a plurality of distributed antenna devicesconnected to the transmitter via a respective physical branch of asignal transmission network comprising a plurality of branches, each ofthe distributed antenna devices comprising at least one antenna which isassociated with a frequency band having a centre frequency f_(c) and acorresponding wavelength λ_(c), the method comprising:

-   -   for at least some of the distributed antenna devices, providing        a respective monitoring device at a distance d<2λ_(c) from the        at least one antenna;    -   transmitting from a central monitoring unit, along the network        and via the distributed antenna device to at least one targeted        monitoring device a monitoring signal;    -   at the at least one targeted monitoring device generating a        response signal and transmitting the response signal to the        central monitoring unit via the distributed antenna device and        the network; and    -   utilizing at least one of the monitoring signal and the response        signal to monitor performance of at least part of the        distributed antenna system.

The monitoring signal may addressed to the at least one targetedmonitoring device by utilizing a respective unique address of the atleast one targeted monitoring device.

The strength of the monitoring signal may be measured at the at leastone targeted monitoring device.

Data relating to the measured strength may then be sent from the atleast one targeted monitoring device via the associated distributedantenna device and the network to the central monitoring unit and thedata may be identified at the central monitoring station by therespective unique address.

The strength of the response signal may be measured at the centralmonitoring unit.

In some embodiments, the strength of the monitoring signal may bemeasured at the at least one targeted monitoring device and the strengthof the response signal may be measured at the central monitoring unitand the results of the measurements may be utilized to monitor thestatus of asymmetrical up and down paths between the central monitoringunit and the at least one targeted monitoring unit.

The method may include the step of utilizing switches which aredistributed in the network selectively to attenuate or divert powerpropagating to at least some of the distributed antenna devices, therebyselectively to switch the at least some of the distributed devices outof the distributed antenna system.

The switches may be controlled by command signals from at least thecentral monitoring unit.

The monitoring signal, the response signal and the command signals maybe at a suitable signal level to ensure communication and sensingbetween central monitoring unit, but below the level of a main signaltransmitted by the central transmitter and below regulatory or operatorrequirements in terms of signal radiated from DADs as to meet regulatoryrequirements or DAS user requirements.

The monitoring signal, response signal and command signals may betransmitted according to a standard protocol and at a level of −30 dBrelative to the main signal.

The monitoring signal, response signal and command signals may betransmitted out of band relative to the main signal.

The method as claimed in the may include the step of indicating themonitored status at the monitoring device and or transmitting statusinformation to other locations or a database to be accessed by variousinterested parties.

The monitored status may be determined by the monitoring device and themonitoring device may then indicate the status. In other embodiments themonitored status may be determined at the central monitoring unit andthen the status may be indicated in response to a command sent from thecentral monitoring unit to the monitoring device.

BRIEF DESCRIPTION OF THE ACCOMPANYING DIAGRAMS

The invention will now further be described, by way of example only,with reference to the accompanying diagrams wherein:

FIG. 1 is a diagram of an example embodiment of a simple distributedantenna system (DAS) and a monitoring system for the DAS;

FIG. 2 is a diagrammatic representation of an example embodiment of adistributed antenna device (DAD) forming part of the DAS in FIG. 1 andan associated monitoring device of the monitoring system;

FIG. 3 is a basic block diagram of an example embodiment of themonitoring device;

FIG. 4 is a basic block diagram of another example embodiment of themonitoring device with an example embodiment of an energy harvestingcircuit for powering the monitoring device;

FIG. 5 is a diagrammatic representation of example embodiments ofmonitoring devices for a multiple-input and multiple-output (MIMO) DAD;

FIG. 6 is a diagrammatic representation of another example embodiment ofa DAS and monitoring system comprising RF path diagnostic devices orswitches and further signal monitoring devices which may be either fixedor roaming; and

FIG. 7 is a block diagram of an example embodiment of an RF pathdiagnostic device or switch.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

An example embodiment of a distributed antenna system (DAS) is generallydesignated by the reference numeral 10 in FIG. 1. The DAS comprises atleast a central transmitter 12 which is connected by a signaltransmission network 14 to a plurality of distributed antenna devices(DAD) 16.1 to 16.n to transmit a main signal propagating in a downwarddirection from the central transmitter 12. The network comprisesphysical branches, such as branches 14.1 and 14.2. Each of the DAD's16.1 to 16.n is connected to a respective sub-branch part or last mile14.11, 14.12, 14.21 and 14.22 of branches 14.1 and 14.2. The network 14may be passive and comprise cables with splitters or may be active asdefined in the introduction of this specification. Hence, the networkmay comprise one or more of frequency converting devices, radio-to-opticand reverse devices, point-to-point wireless components andbidirectional amplifiers.

An example embodiment of a DAD 16.1 is shown in FIG. 2. Each DADcomprises at least one antenna 18. The antenna is associated with afrequency band having a centre frequency f_(c) and an associatedwavelength λ_(c). The DAD may optionally comprise a radome 20.

An example embodiment of a monitoring system is generally designated bythe reference numeral 22 in FIG. 1. The monitoring system comprises acentral monitoring unit CMU 24 which is coupled to the network 14. Atleast one monitoring device 28.1 to 28.n is associated with at least oneof said distributed antenna devices 16.1 to 16.n and permanently mounteda distance d (see FIG. 2) away from the antenna of the associateddistributed antenna device. The distance d is less than 2λ_(c),preferably less than λ_(c), more preferably less than λ_(c)/2 and evenless than λ_(c)/4. Referring to FIG. 3, the monitoring device comprisesa local controller 30, a transceiver 32 and an antenna 34. The localcontroller being configured, upon being polled by the CMU 24 with amonitoring signal in a downward direction and along a downward path viathe network 14 and the distributed antenna device 16.1, to cause thetransceiver 32 to respond to the monitoring signal by transmitting aresponse signal in an upward direction and along an upward path to theCMU 24 via the distributed antenna device 16.1 and the network 14. Insome embodiments, especially in active networks, the upward and downwardpaths may not be the same and may hence be asymmetrical.

The CMU 24 may comprise a transceiver which may be connected to thenetwork 14 by coupler 26 in a region of the network 14 towards thecentral transmitter 12 and before or upstream of a first branch 14.1,14.2.

Referring to FIG. 3 and as stated above, the monitoring device 28.1comprises the local controller 30 which is connected to the transceiver32 and which is connected to poles 34.1 and 34.2 of a broadband dipoleantenna 34. Each monitoring device may be associated with a uniqueaddress which is stored in a memory arrangement of the controller. Theaddress may be used by the CMU 24 to poll or interrogate individualmonitoring devices. The address may also be used to identify responsesreceived by the CMU 24 from monitoring devices by virtue of themonitoring device appending its unique address to a response signal sentback to the CMU 24.

The CMU 24 may use any suitable communication standard to communicatewith or poll via the network 14 any one or more of the monitoringdevices 28.1 to 28.n. As an example, the ZigBee communication protocoland devices operating in the 2.4 GHz licensed frequency bands may beused to effect communication and addressing between the CMU 24 and otherdevices of the monitoring system and the same signals may additionallybe used to monitor RF path and DAD operation. The ZigBee units mayadditionally be equipped with the ability to generate test signals atother frequencies used in the DAS system, if required. Other protocolsoperating at other frequencies may serve a similar purpose, such asBluetooth, Wifi and/or similar communication protocols.

As shown in the example embodiment in FIG. 4, the monitoring device 28.1may comprise an indicator arrangement 36, for example in the form of oneor more LED's which may be used to indicate a status of the DAD 16.1, asexplained below. Each monitoring device may comprise means (not shown)for measuring the strength of a received monitoring signal. Furthermore,the monitoring device may comprise a power supply comprising a battery40, alternatively a battery plus an energy harvesting circuit, which mayfor example comprise diode 42 and capacitor 44. In some embodiments, themonitoring device 28.1 may be encapsulated in any suitable encapsulation38, for example a flexible sleeve or envelope, which may be adhered to asurface of the radome 20. This may be particularly advantageous whenretrofitting monitoring devices on existing DAD's comprising radomes.

The CMU 24 may comprise or be connected to a database (not shown) forstoring said unique addresses, monitored status of each DAD as well asdata relating to the position of the associated DAD. Hence, themonitoring devices may comprise complementary circuitry to respond viathe network 14 to the CMU 24. The CMU 24 may also comprise means formeasuring the strength of a response signal received from any of themonitoring devices and processing means for processing suchmeasurements.

In order not to interfere with the DAS, the CMU 24 is coupled to themain branch of the network 14 via coupler 26 to transmit via the networkweak monitoring signals (at say −10 dB of the main signal, preferably−30 dB) and/or out of band monitoring signals which are received andprocessed by the monitoring devices 28.1 to 28.2 as will be describedbelow.

In FIG. 5, there is shown an example embodiment of a multiple-input andmultiple-output (MIMO) DAD 116 comprising a first antenna 118.1 and asecond antenna 118.2 which are orthogonally polarized. Associatedmonitoring devices 128.1 and 128.2 with corresponding antennaorientations are provided in close proximity d (as defined above) fromthe first and second antennas, to ensure that both can be monitoredindependently. If the number of monitoring devices is equal to thenumber of MIMO elements in a MIMO DAD and close enough to the respectiveassociated elements, the monitoring system may be used to monitor statusof specific MIMO antennas within a DAD.

The CMU 24 may be pre-programmed to poll each monitoring device 28.1 to28.n on an intermittent, alternatively periodic basis. This is done bysending the monitoring signal with each monitoring device's addresssuccessively and waiting for the response signal from the monitoringdevices. Upon receiving the response signal from a monitoring device,the strength of that response signal is measured, compared to areference and/or previous values and stored and/or communicated to anexternal control centre. If no response is received from a monitoringdevice, after a predetermined number of polls, the monitoring deviceand/or DAD is tagged as defective. If a signal strength measurement on aresponse signal indicates degradation in or to the transmission pathbelow a predetermined limit, then that monitoring device is instructedfrom the CMU 24 to update locally its status to “low level” and theindicator means 36 is caused to indicate that status. Further forexample, failure of both devices 28.1 and 28.2 could be interpreted bythe CMU 24 as likely failure of branch 14.1, rather than failure of DAD16.1 and 16.2.

The monitoring device may be programmed to activate the local indicator36 or may be configured in response to a command signal from the CMU 24to display status, depending on where the measurement is done, at themonitoring device or at the CMU 24. Status indications may include:failure, power reduction below predetermined limit, working status, lowbattery etc.

Hence, the monitoring devices may use the polling or monitoring signalreceived via its associated DAD to measure the signal quality and reportback by means of the response signal such signal quality to the CMU 24.Alternatively, the strength of the response signal as measured at theCMU 24 may be used to determine the path quality between the DAD and theCMU 24. Whether sensing is done at the CMU 24 or at the respectivemonitoring devices, the monitoring system 22 may be configured to senseboth a) failure vs operational and b) relative signal level.

As explained in more detail below, additional components which operateon the same protocol as the CMU 24 and the monitoring devices 28.1 to28.n may be inserted into the network 14, to adjust networkconfigurations or parameters. Such components could be inserted in-lineto switch off certain DAD's or branch lines leading to DAD's or toattenuate/increase signal levels to DAD's or sections of DAD's.

Tones may be inserted at different frequencies used by the DAS 10 byeither monitoring devices 28.1 to 28.n or CMU 24, to enable in-band orband related and/or more accurate measurements on the system 10.

Monitoring devices 28.1 to 28.n could use wireless mesh or othercommunication to adjacent or closely located other monitoring devices toestablish an alternative response or up path back to the CMU 24, in theevent of failure of a line or branch to which they are connected.

FIG. 6 is a figure similar to FIG. 1, but with RF path diagnosticdevices or switches 50.1 to 50.n included in at least some of thebranches of the network. The switches may be used selectively to switcha major part of signal power (for example reducing the through signal by30 dB and diverting the major part to a suitable dummy load) as will bedescribed below. Additionally, some further distributed signalmonitoring units 60.1 to 60.n are included. These may be stationary ormobile.

FIG. 7 illustrates an example embodiment of switch 50.1. The switch mayhave a first port 52 for a path part extending to CMU 24 and a secondport 54 for a path part extending to at least one DAD, such as DAD's16.1 and 16.2. The switch is connected to a controller 56 and to a dummyload 58. The controller 56 is in communication with the monitoringsystem 22 using the above communication protocol. Path 57 carries acontrol signal and optionally some suitable reduced power tap off fromthe main line signal, to allow communication to the monitoring system.Controller 56 is configured upon command from the monitoring system orautomatically under program control, to either allow the DAS signal toflow between ports 52 and 54 or a major part thereof to flow to dummyload 58, with say only −30 dB of the original signal to pass from thefirst to the second port. The small signal still passed allowscommunication to happen between monitoring units 28.1 to 28.n and CMUunit 24 while effectively isolating specific DAD's or groups of DAD's,to identify problems such as passive intermodulation (PIM) or performpropagation tests from specific DAD's or groups of DAD's, while othersare effectively non-operational.

Hence, the system may comprise path diagnostic devices or switches 50.1to 50.n comprising respective controllers 56 operating on the samefrequency and protocol as the monitoring devices and CMU 24. Thecontroller 56 is operative (under program control or on command from theCMU 24) to cause the switch to switch between a first state wherein theswitch allows the RF signal to continue unhindered and a second statewherein the switch causes part of the RF signal to be diverted to thedummy load 58, so that most power goes to the dummy load with less than−10 dB, but preferably −30 dB continuing along the branch to which it isconnected. This switch can be addressed by either the CMU 24 or anymonitoring device or any other transmission diagnostic device, since alloperate on the same frequency and uses the same communication protocol.Hence, such switches may be used to selectively isolate parts of the DASsystem or specific DADs for diagnostic purposes or to test themonitoring device operation.

Furthermore, the system may be configured to measure signal in bothdirections of the DAS path to a specific DAD. The signals along the“down path” (that is towards the DAD's) and the “up path” (that istowards the CMU) may not be symmetrical due to active components.Utilizing the signal sensing and transmitting capabilities of themonitoring devices and the CMU 24, the measurements could be done for“up” and “down” paths. Such measurements allow faults with specificunits (up or down amplifiers for example) to be pinpointed by comparisonof the signal measurements along both paths and associated with specificDAD's or group of DAD's.

Still furthermore, the monitoring system 22 may in addition to themonitoring devices 28.1 to 28.n and the path diagnostic devices 50.1 to50.n comprise further distributed signal monitoring devices 60.1 to 60.n(shown in FIG. 6) and which may be similar to that of the prior art.However, the distributed signal monitoring devices 60.1 to 60.n mayoperate on the same communication protocol and frequency band as themonitor system 22. The signal monitoring devices 60.1 to 60.n may havethe same functionality as the monitoring devices 28.1 to 28.n, but mayhave different form factors and/or antenna configurations and/or energyharvesting circuitry (such as photovoltaic convertors). These devices60.1 to 60.n may be scattered or distributed through the DAS coveragearea in permanent locations or alternatively temporarily locatedthroughout the area to validate correct DAS system coverage afterinstallation, or, may be moved around through the DAS coverage areaafter final installation to monitor signal and use DAD location eithercorrelated in real time or by correlating mobile position which may betime related by mobile unit during walk-through or other mobile test ofcoverage. During such measurements, communication (polling) signals andunique identifiers associated with DAD's and further monitoring deviceswill be used in either up or down or both directional measurements todetermine coverage.

Communication may happen between the CMU 24 via DAD's to signalmonitoring devices 60.1 to 60.n directly or may also be relayed viamonitoring devices 28.1 to 28.n. Monitoring may be performed usinginformation from signal monitoring device 60.1 to 60.n operation inconjunction with information from monitoring device 28.1 to 28.noperation and optionally selective isolation using path diagnosticdevices or switches 50.1 to 50.n, to get detailed information ofcoverage.

The invention claimed is:
 1. A monitoring system for a distributedantenna system comprising at least a central transmitter and a pluralityof distributed antenna devices connected to the transmitter via arespective physical branch of a signal transmission network comprising aplurality of branches, each of the distributed antenna devicescomprising at least one antenna which is associated with a frequencyband having a centre frequency fc and a corresponding wavelength λc, themonitoring system comprising: a central monitoring unit which isconfigured to be coupled to the network; at least one path diagnosticswitching device configured to be connected in a branch of the networkand further configured to be switched between a first state wherein itpasses along branch input power received by the at least one pathdiagnostic switching device and a second state wherein it attenuates ordiverts away from the branch at least some of the input power receivedby the at least one path diagnostic switching device; at least onemonitoring device associated with at least one of said distributedantenna devicesand permanently mounted a distance d away from the atleast one antenna of the distributed antenna device, the distance dbeing less than 2 times λc (2λc); the at least one monitoring devicecomprising a local controller, a transceiver and an antenna; and thelocal controller being configured to, upon being polled by the centralmonitoring unit with a monitoring signal via the associated distributedantenna device and the respective branch of the network, to process themonitoring signal to measure the quality of the monitoring signal and tocause the transceiver to respond to the monitoring signal bytransmitting a response signal comprising an indication of the measuredquality to the central monitoring unit via the associated distributedantenna device and the network.
 2. The monitoring system as claimed inclaim 1 wherein the at least one monitoring device is mounted a distanced away from the at least one antenna, the at least one antennaassociated with a frequency band having a centre frequency fc and acorresponding wavelength λc, and wherein d is less than (λc/2).
 3. Themonitoring system as claimed in claim 1 wherein the at least onemonitoring device comprises a local power supply comprising at least oneof a battery and an energy harvesting circuit for collecting configuredto collect energy from the distributed antenna device through theantenna of the monitoring device.
 4. The monitoring system as claimed inclaim 1 wherein the at least one monitoring device is associated with arespective unique address which is stored in a memory arrangement of themonitoring device.
 5. The monitoring system as claimed in claim 4wherein the central monitoring unit comprises means for measuring the aprocessor configured to process a measured strength of the responsesignal received from the at least one monitoring device and be connectedto a database for storing data relating to the unique addresses of theat least one monitoring device and at least one of data relating to thea monitored status of the associated distributed antenna device and datarelating to a position of the associated distributed antenna device. 6.The monitoring system as claimed in claim 1 wherein the at least onemonitoring device comprises an indicator arrangement for providing ahuman perceivable configured to provide an indication relating to amonitored status of at least one of the associated distributed antennadevice and the branch connected thereto, based on the measured qualityof the monitoring signal.
 7. The monitoring system as claimed in claim 1wherein the at least one monitoring device is mounted in or on theassociated distributed antenna device.
 8. The monitoring system asclaimed in claim 7 wherein the at least one monitoring device isencapsulated in a housing which is mountable on an optional a radome ofthe associated distributed antenna device.
 9. The monitoring system asclaimed in claim 1 wherein the central monitoring unit is configured tobe coupled to the network by a coupler to inject the monitoring signalinto the network.
 10. The monitoring system as claimed in claim 1wherein the monitoring signal and the response signal are at least oneof a) at least 10 dB weaker than a main signal transmitted by thecentral transmitter and b) out of band relative to the main signal. 11.The monitoring system as claimed in claim 1 comprising at least one pathdiagnostic switching device which is connected in a branch of thenetwork and configured to be switched between a first state wherein itpasses along the branch input power received by the switch device and asecond state wherein it attenuates or diverts away from the branch atleast some of the input power received by the switch device.
 12. Adistributed antenna system comprising a monitoring system as claimed inclaim
 1. 13. A The monitoring system as claimed in claim 1 wherein thequality of the monitoring signal is measured by measuring the strengthof the monitoring signal and wherein the indication of the measuredquality comprises data relating to the measured strength.
 14. A methodof monitoring performance of a distributed antenna system comprising atleast a central transmitter, at least one path diagnostic switchingdevice, and a plurality of distributed antenna devices connected to thetransmitter via a respective physical branch of a signal transmissionnetwork comprising a plurality of branches, each of the distributedantenna devices comprising at least one antenna which is associated witha frequency band having a centre frequency fc and a correspondingwavelength λc, the method comprising: for at least some of thedistributed antenna devices, providing a respective associatedmonitoring device at a distance d<2λc from the at least one antenna;switching the at least one path diagnostic connected in a branch of thenetwork between a first state wherein it passes along branch input powerreceived by the at least one path diagnostic switching device and asecond state wherein it attenuates or diverts away from the branch atleast some of the input power received by the at least one pathdiagnostic switching device; transmitting from a central monitoring unitto at least one targeted monitoring device a monitoring signal via thean associated distributed antenna device and the respective branch ofthe network; and atby the at least one targeted monitoring device,processing the monitoring signal to measure the quality of themonitoring signaland, generating a response signal comprising anindication of the measured quality and transmitting the response signalto the central monitoring unit via the associated distributed antennadevice and the network.
 15. The method as claimed in claim 14 whereinthe monitoring signal is addressed to the at least one targetedmonitoring device by utilizing a respective unique address of the atleast one targeted monitoring device.
 16. The method as claimed in claim14 wherein the monitoring signal is processed by measuring the strengthof the monitoring signal at the at least one targeted monitoring device.17. The method as claimed in claim 16 wherein data relating to themeasured strength is sent from the at least one targeted monitoringdevice via the associated distributed antenna device and the network tothe central monitoring unit and wherein the data is identified by the arespective unique address of the at least one targeted monitoringdevice.
 18. The method as claimed in claim 17 wherein the strength ofthe response signal is measured at the central monitoring unit andwherein one or more results of the measurements are utilized to monitorthe status of asymmetrical up and down paths between the centralmonitoring unit and the at least one targeted monitoring unit.
 19. Themethod as claimed in claim 14wherein distributed switches in the networkare utilized, further comprising, by the monitoring unit, controllingdistributed switches in the network to selectively to attenuate ordivert power propagating to at least some of the distributed antennadevices, thereby to switch the at least some of the distributed devicesout of the distributed antenna system, and wherein the switches arecontrolled by command signals from at least the central monitoring unit.20. The method as claimed in claim 19 wherein the monitoring signal, theresponse signal and the command signals are at a signal level to ensurecommunication and sensing between central monitoring unit, themonitoring devices and the switches devices distributed switches, butbelow the a level of a main signal transmitted by the centraltransmitter.
 21. A monitoring system for a distributed antenna system,the monitoring system comprising: a central monitor configured to becoupled via a network to a plurality of antennas; and a plurality ofremote monitors associated with the plurality of antennas, the pluralityof remote monitors comprising: a first remote monitor of the pluralityof remote monitors configured to, upon being polled by the centralmonitor with a first monitoring signal transmitted via the network and afirst antenna associated with remote monitor, transmit a first responsesignal to the central monitor via the first antenna associated with thefirst remote monitor and the network; and a second remote monitor of theplurality of remote monitors configured to, upon being polled by thecentral monitor with a second monitoring signal transmitted via thenetwork and a second antenna associated with the second remote monitor,transmit a second response signal to the central monitor via the secondantenna associated with the second remote monitor and the network;wherein the first remote monitor and the second remote monitor areassociated with the first and second antennas sharing a branch of thenetwork connecting the first and second antennas to the central monitor;wherein the central monitor is further configured to: in response to adetermination that no first response signal has been received from thefirst remote monitor, label at least one of the first remote monitor orthe first antenna associated with the first remote monitor as defective;and in response to a determination that a signal strength of the firstresponse signal received from the first remote monitor does not satisfya first signal strength threshold and that a signal strength of thesecond response signal received from the second remote monitor does notsatisfy a second signal strength threshold, determine that the branchhas failed.
 22. The monitoring system as claimed in claim 21 wherein thefirst response signal comprises a first unique identifier of the firstremote monitor and the second response signal comprises a second uniqueidentifier of the second remote monitor, and wherein the central monitoris configured to identify the first response signal using the firstunique identifier and the second response signal using the second uniqueidentifier.
 23. The monitoring system as claimed in claim 21 wherein atleast one of the first or second monitoring signal is at least one of:out of band relative to a main signal transmitted by a centraltransmitter of the distributed antenna system or at least 10 dB weakerthan the main signal.
 24. The monitoring system as claimed in claim 21wherein the central monitor is further configured to, responsive toreceiving the first response signal from the first remote monitor and adetermination that a signal strength of the first response signalreceived from the first remote monitor does not satisfy a signalstrength threshold, transmit an instruction to the first remote monitorto change status of the first remote monitor and provide indication ofthe change.
 25. The monitoring system as claimed in claim 21 wherein thecentral monitor is further configured to: in response to a determinationthat no second response signal has been received from the second remotemonitor, label at least one of the second remote monitor or the secondantenna associated with the second remote monitor as defective.